On equuality testing protocols and their security

Sammanfattning: This thesis is written for the Swedish degree Licentiate ofScience, Teknisk Licentiat.It is a university degree, between that of master andthat of doctor.The main focus of the thesis is on the construction ofsecure protocols for comparing the underlying plain-texts inElGamal encryptions. The protocols make use of the malleabilityof the ElGamal encryption scheme. More specifically they usethe multiplicative homomorphic property of ElGamal.We present fully verifiable protocols for both thetwo-party setting and the multi-party setting. These protocolsare built on sub-protocols, which are specially constructed tofit the present setting. We also present full proofs forcompleteness, soundness, and zero-knowledge for all the givenprotocols, in the random oracle model.